The risk of cancer in populations exposed to ionizing radiation is evaluated among Japanese A-bomb survivors, patients given therapeutic or diagnostic radiation, occupational groups, and persons exposed to naturally occurring radiations. Biochemical approaches are incorporated into studies to help define basic mechanisms. Program members serve on committees advising the government and international agencies. Results of studies suggest that (1) indoor exposure to radon may be less risky than previously believed; (2) diagnostic x-ray exposure is unlikely to be a significant cause of leukemia or lymphoma in our society, although excessive exposure may increase the risk of multiple myeloma; (3) living near nuclear facilities in the United States is not associated with a detectable increased risk of childhood leukemia or any other cancer; (4) the risk of radiation-induced breast cancer declines with increasing age at exposure, the dose response is linear, and risk remains for at least 50 years; (5) exposure prior to first pregnancy carries a higher breast cancer risk than exposure afterwards; (6) radiotherapy for breast cancer increases the risk of leukemia, especially in combination with alkylating agents; (7) low-dose radiotherapy to treat uterine bleeding induces many more leukemias than high-dose radiotherapy to treat cervical cancer or uterine cancer; (8) high-dose radioactive iodine treatments did not increase the risk of leukemia, suggesting that protraction of dose reduces risk; (9) high-dose radiotherapy in childhood increases the risk of thyroid cancer; (10) radiotherapy for bilateral retinoblastoma greatly increases the risk of osteosarcoma, indicating the importance of a genetic susceptibility in radiation risk.